Semiconductor Optical Amplifiers (SOAs) have been shown to operate as modulators in applications for fiber optical communications. However there has been minimal work done in the area of applying SOAs to operate as modulators for other applications and much of that previous work has been related to phase modulation. For example, Mørk et al. (The Modulation Response of a Semiconductor Laser Amplifier,” Jesper Mørk, et al., IEEE Journal of Selected Topics in Quantum Electronics, Vol. 5, May/June 851, 1999) analyzed the performance of SOAs as amplitude modulators for small-signal analog modulation frequencies greater than 10 GHz.
For other applications, such as coherent imaging, however, high extinction ratio and narrow linewidth are critical and involve operating the SOA under large signal conditions while still minimizing the “chirp” added to the linewidth by the amplifier. Extinction ratios greater than 20 dB are required for good system performance.
Comparable modulator components such as acousto-optic modulators (AOMs), electro-absorption modulators (EAMs), and electro-optic phase modulators used in an interferometer configuration (e.g., such as a Mach-Zehnder interferometer) have been used as amplitude modulators. Acousto-optic modulators use high frequency acoustic signals to modulate the refractive index of an acoustically active material such as lithium niobate, to create a diffraction grating that can scatter the light signal and, hence, modulate its amplitude. The disadvantage of AOMs is that they require high power and high frequency acoustic signals to allow high speed modulation. These factors increase the cost and complexity of the drive electronics. Additionally, AOMs can be bulky and hard to utilize in miniaturized systems.
Electroabsorption modulators are semiconductor heterostructure devices that operate by voltage shifting of the semiconductor optical bandgap using the Franz-Keldysh effect or quantum confined Stark effect. EAMs designed in a waveguide configuration can have very high modulation bandwidths, of the order of tens of GHz. However, because of the need to couple light in and out of the edges of the waveguide, they can have high coupling losses. For example, when fiber coupled, the fiber-to-fiber insertion losses can exceed 5 dB for wavelengths of 1550 nm, and sometimes approach 15 db, especially for visible wavelengths where the fiber optic spot size and the semiconductor waveguide are much smaller. See, for example, “Low Insertion Loss and Low Dispersion Penalty InGaAsP Quantum-Well High-Speed Electroabsorption Modulator for 40-Gb/s Very-Short-Reach, Long-Reach, and Long-Haul Applications,” Won-Jin Choi, et al., Journal of Lightwave Technology, Vol. 20, Issue 12, p. 2052, 2002.
Thus, high power seed lasers or subsequent amplification are required to make up for the lost optical signal. In addition, the extinction coefficient of EAMs is typically not very high, reaching approximately 11 dB for waveguide devices with high RF bandwidths (Id.). Mach Zehnder or other interferometer style amplitude modulators suffer from the same issues, plus the chip designs are typically more complex; requiring multiple waveguides and branching and combining junctions.
Thus there is a need for an absorption modulator design that can provide high extinction ratios and good coherence properties while minimizing insertion loss.